It is of course known to measure vehicle acceleration by means e.g. of some form of accelerometer, or alternatively by measuring the speed of the vehicle wheels and differentiating with respect to time. Unfortunately the vehicle, due to its suspension etc., and the driveline, due to its compliance, have dynamics exhibiting resonance at frequencies which can be as low as 2 Hz in motor cars and still lower in larger vehicles. This can create corresponding oscillation in measured acceleration signals. Signal noise can also be a problem. The signal can be filtered to improve its quality but a filter with a time constant long enough to remove the low frequency oscillation would introduce an appreciable time lag.
The problem is experienced in connection with electronic systems for control of vehicle powertrains. The present invention has in fact been developed for use in a system which controls a powertrain using a continuously variable transmission of so-called “torque controlled” type (the term is known in the art and transmissions of this type have for example been described in European patent 832376 and its US counterpart U.S. Pat. No. 6,071,209, both granted to Torotrak (Development) Limited). In such transmissions variator ratio is not directly set, but instead transmission ratio is able to change in accordance with changes in engine and vehicle speed. To determine rate of ratio change, vehicle acceleration is required. The rate of ratio change is needed for various purposes in controlling the powertrain. If a simple low pass filter were used with a long enough time constant to remove the low frequency oscillation from a measured value of vehicle acceleration, the speed of response of the control system would be unacceptably compromised.